1. Field of the Invention
The invention relates to a process for producing a semiconductor device and the semiconductor device, and more specifically, relates to the gettering of impurities in an SiC substrate for forming a semiconductor device.
2. Description of Related Art
To prevent degradation of semiconductor characteristics by metallic impurities in a semiconductor crystal of Si, etc., a strain region is created by ion implantation, laser irradiation, forming a film of a different material, and the like, to provide a gettering center for the impurities.
As a problem unique to SiC monocrystals, the inventors have discovered that when a heat treatment is carried out thereon at a high temperature, particularly a temperature of 1500° C. or higher, impurities such as metals that are either contained in the crystal or lie adjacent thereto diffuse through the crystal.
Impurities are always present in SiC semiconductor monocrystals that are used as practical materials. More specifically, when SiC monocrystals are grown by the solution method, metal elements are often added to the SiC solution to increase the growth rate, improve crystallinity, and the like. For example, Japanese Patent Application Publication No. 2007-261844 (JP-2007-261844 A) discloses the addition of Cr, and Japanese Patent Application Publication No. 2009-426770 (JP-2009-126770 A) discloses the addition of Ni to the SiC growth solution, and in some cases these metals remain as impurities in the grown SiC monocrystal. When an epitaxial layer for the device region is formed on such an SiC substrate produced by the solution method, the diffusion of the metal impurities from the SiC substrate into the epitaxial layer will degrade the properties of the device.
Therefore, the amounts of impurity elements adversely affecting conductive and device properties that are incorporated into the crystal must be reduced and those impurity elements that do become incorporated must be contained outside the device region. In the case of a Si crystal, alkali metal elements such as Na, K, Li, etc., and heavy metal elements such as Fe, Cu, Au, Cr, etc., are taken into account. It is known that alkali metal elements affect adversely metal-oxide-semiconductor (MOS) characteristics, and heavy metal elements cause shortening carrier life and oxidation stacking fault (OSF) defects. Therefore, the gettering (capture and immobilization) of these residual impurity elements by the use of a strained layer that is intentionally formed in a specific part of the crystal (outside the device region) is already available in the case of Si crystals.
In the case of an SiC crystal, on the other hand, thermal diffusion is not considered to occur substantially at the kind of heat treatment temperature (maximum of about 1200° C.) than is normally used for a Si crystal because the diffusion coefficient of the impurities is extremely small (Yu. A. Vodakov and E. N. Mokhov, “Silicon Carbide” 1973 (Univ. South Carolina Press, Columbia, 1974, p. 508)). Therefore, almost no reports concerning gettering technology in SiC crystals can be found.
With this new knowledge, however, the inventors have ascertained that when a high temperature heat treatment (1500° C. or higher) specific to SiC is carried out, even heavy metal elements such as Cr and Ni will diffuse through the SiC crystal even though the diffusion coefficients of the impurities are low compared with Si. Therefore, these elements must be taken into account, and it has become necessary carry out a gettering treatment on SiC crystals.
Japanese Patent Application Publication No. 2007-251172 (JP-20077-251172 A) discloses using He and H ion beams to form an ion-enriched layer of a given depth on an SiC substrate or a Si substrate, and then conducting gettering to trap contaminants such as heavy metal ions by carrying out a heat treatment at a temperature that does not exceed a critical temperature whereat destruction of the substrate occurs. Unlike Si, however, in SiC, the diffusion of contaminants does not easily occur and it is difficult to achieve recovery of the substrate after gettering. Therefore, as an issue specific to SiC, it is necessary to focus on the sites on the substrate and the manufacturing processes that require gettering, and to eliminate, unnecessary process steps. However, JP-2007-251172 A does not disclose an example wherein gettering is carried out on SiC, and it contains no statements or implications relevant to this issue.
Japanese Patent Application Publication No. 2010-098107 (JP-2010-098107 A) discloses that in a semiconductor apparatus used as an imaging device, a gettering layer of a desired depth was fabricated by irradiation with a laser beam from the side opposite the epitaxial layer, which is the structural element for forming the circuit layer, etc.
Japanese Patent Application Publication No. 2006-231430 (JP-2006-231430 A) discloses setting the temperature for forming a gettering layer at 1100° C. to 1300° C. when carrying out gettering of heavy metals.
Japanese Patent Application Publication No. 58-147126 (JP-58-147126 A) discloses forming a gettering layer on the underside of a semiconductor substrate by carrying out a heat treatment at a temperature of 1050° C. or higher to diffuse supersaturated interstitial oxygen therethrough.
Japanese Patent Application Publication No. 07-201971 (JP-07-201971 A) discloses fabricating a plurality of semiconductor device sections on a substrate and forming a gettering site on the sides opposite the divided sections of the semiconductor devices that also serve as the device, formation regions of the semiconductor substrate.
However, none of this related art assumes the diffusion of contaminating metallic elements that are either present in the substrate crystal or infiltrate from outside the substrate crystal in a SiC semiconductor substrate, and they have not noticed the need of a gettering layer therefor, particularly when a heat treatment at 1500° C. or higher is carried out.